Aircraft navigation systems



Nov. 1l, 1958 s. KELLoGG 2ND AIRCRAFT NAVIGATION SYSTEMS Filed Aug. 31, 1955 l NVE NTO R fit-N02@ L 066,2# /f/ M ATTORNEY United States Patent() f' AIRCRAFT NAVIGATION SYSTEMS Spencer Kellogg 2nd, Glen Head, N. Y., assignor to Sperry Rand Corporation, a corporation of Delaware Application August 31, 1955, Serial No. 531,673

7 Claims. (Cl. 343-107) The present invention relates generally to aircraft navigation systems and more particularly to such systems by means of which a craft may be controlled so as to `seek and thereafter to be maintained on a prescribed or` Y vIpatents are assigned to the same assignee as my present invention. Y

In my patents is disclosed a navigation system by means .of which a craft may be controlled, in one mode of J .operation thereof, to seek or turn toward a preselected heading from an existing heading and thereafter to be maintained on said preselected heading, wherein the craft is controlled in accordance with the algebraic sum of signals representative respectively of the angular displacement between the existing heading of the aircraft and the preselected heading and a signal proportional to the roll angle or the roll attitude of the craft, the former signal being proportional to a displacement term and the latter signal being proportional to the rate of change of such displacement, whereby if the algebraic sum of these two signals is maintained substantially at` zero, an asymptotic approach to the selected heading will be executed. ln another mode of operation of a navigation system disclosed in my above-noted patents the craft may be controlled so as to seek a radio-defined course or flight path wherein the craft is controlled in Yaccordance with the algebraic sum of signals representative of the displacement of a directional craft from the radio beam as may be measured by means of `a suitable radio receiver, a heading error` signal representative of the angle betweenthe heading of the aircraft with respect to the compass bearingof the beam which may be pro- `,duced for example by means of a stabilized magnetic .compass, and a signal proportionalto the roll angle or rollV attitude of the craft as `produced for example by means of a vertical gyroscope. These three signals are respectively proportional to the displacement, rate, and `acceleration ofthe craft with respect to the radio beam.

",As described therein, the combined radio displacement and radio rate signals serve to provide an asymptotic `approach to the beam while the acceleration signal serves to provide anticipation for enabling the craft to be maintained on the asymptotic aproach when the same is` conftrolled in accordance with the combined signals. In the above-mentioned Hassler et al.`patent there is disclosed .la system of this general character wherein the rate and acceleration `signals are derived directly from the radio A,displacementsignalrather than `from the compass and vertical gyro asin the patents, thus improving the per- ,..formance ofthe craft under adverse wind conditions and 2,860,335 Patented Nov. 11, 1958 ICC under varying sensitivities of the beam displacement signal, i. e. whether the aproach is being made at a distance far away from the radio transmitter or close thereto and also under conditions wherein the radio displacement signal is subject to ramdom perturbations caused by bends therein due to local geography, etc. In the above-mentioned Hassler et al. patent a system is disclosed by means of which a noisy signal or randomly fluctuating signal may be smoothed bypassing the signal through a smoothing circuit such as, for example, a low pass filter and wherein means are provided for compensating for the phase lag brought about by .such a filter such that the higher derivatives subsequently derived from the smoothed version of the noisy signal has substantially the proper 90 and 180 phase relation with respect to the noisy displacement signal itself. Several forms of apparatus for accomplishing the above are set forth in this patent and it is the object of the present invention to provide another form of apparatus for deriving a smoothed version of an inherently noisy signal, particularly a smooth beam acceleration signal for use in a system of the above character. i lt is the principal object `of the present invention to provide a` simplified flight navigation system of the character set forth in the above'noted patents Whicheffects not only a reduction in the cost, weight, and size of the equipment, but also an improvement in the performance thereof. i

ln the above-noted patents a roll displacement signal is used as an approximation for a craft rate of turn signal which signal, in turn, is proportional to the acceleration of the craft toward a radio beam defined liight path. However, by replacing the vertical gyro with rate of turn gyro, a rate of turn signal may be directly obtained and this signal will be correct regardless of craft airspeed. However, it is known that the output of a rate of turn gyro is inherently noisy unless heavily damped, i. e. it is subject to random perturbations brought about primarily by wind gusts acting on the craft. Heretofo'e these random perturbations have been considerablyi'educed by heavily damping the rate gyro or by increasing the retraint imposed about the gyro Vprecesson axis. However, this meansV of removing unwanted noise in the gyro output results in a decreased sensitivity or response of the gyro to craft turning rates. Another method of 'removing the noise from the rate of turn signal wouldbe `tering technique imposes a lag in the signal, and, unless compensated, the rateof turn signal will not be in the proper phase relation with the radio displacement signal and/or the radio rate signal. In order to compensate for the lag in the signal produced by such filtering, the technique of rate compensation as set forth in the abovenoted patent application is employed. In accordance with the teaching of the present invention a signal proportional to roll rate is provided. This roll rate signal `is proportional to the rst derivative of rate of turn, i.`e. it leads the rate of turn signal by 90. Thus by selecting the proper amount of lead signal, the combination thereof with the noisy signal will produce a resultant or net signal that leads the noisy signal by the amount of the lag anticipated as a result of the filtering or smoothing process. In this manner a smooth output signal, in the present case a smooth rate of turn signal that is substantially in the proper phase relation with respect to the p other signals in the system will be obtained.

A further object of the` present invention is to provide means for deriving a craft rate-of "turn signal which is characterized by its highy response to craft rates of turnv and at the same time has a high degree of stability. n lt is a furtherY object v of the present invention to pro-A A oftjn'rnsignal whichis" derived from signal co pone s; proportional to craft rate of change of 'nd rate of change of roll angle in which only th'elow fregency components of each are employed, tlehigh freqiiency components thereof being attenuated. Another objece of theipresent invention is to provide s'ooth rate of turn signal for aircraft control purbysr'neans fof a rate gyro mounted in the aircraft afa inclined position with respect tothe craft fore and ffaxis soh that the saine is responsive both to rate of change of craft heading and rate of change `of roll le and wherein a signal generatormounted on the p cessionl airis ofthe Y",gyro is responsive to both said rates f5 craft movement, the resultant thereof having a leading phase -characteristic with respect to craft rate of 'change' of heading alone, and wherein 'this resultant signal-aisv 'passedthroughla low'pass filter having a time onstnt-- 'suchl that. the undesired higher frequency componrits o'f'tsaid resultant signal are attenuated While allowing/the vdesired low frequency components thereof tofp`ss',-'lthe-angle'of inclination ofthe 'gyro being proportional to the time lag or time constant of the filter. Gtherlobjects and advantagesI of the present invention notila 'this time morexparticularly enumerated will befv oonielappaient asfthe description of a preferred embodi-v i'rrierit o'ffthe presentrinventionV proceeds, reference being made1to..theaccorrpanying. drawings wherein: Fig. l isV a schematic illustration of the present inventionlasapplied. tolan aircraft navigation system.

'^Eig.a.2. isalvector diagram serving to illustrate the :phasefilaguelfectoflter smoothing and the compensation, o'flthis effect; and v Fig. 3 illustrates a modification of the apparatus illus-Y `trated.in. Fig.j1. [.A`sstated, .the.nayigation system disclosed in Fig. 1 .is,.of the..type shown inthe above-mentioned patents -Lwherein signalsV proportional to the displacement of an aircraft from a 'radio-denedpath and signals proporetiohaltoitherate ofchangefand acceleration of saidv disl placement are provided for enabling the craft to be flown gouliovaluesgof, the termsof the equation, providing an ,.outpuptlwhichthen serves to provide an indication to the :.fpilot of .the 4correctattitude of the craft toA satisfy such '.egufa 'on atganyparticular instant. ,lf the instantaneous Y n oftheequation issatisedlall terms thereofwill lmatelynbe, reduced. to zeroL thereby providing the 5:51j1f ect,,navigationginforrnationV for the craft. In otherl rdsthe s yst`e,m -,herein used, for exemplary purposes, furnishesinformation tothe pilot whereby he knows spiare 4i,order to-cause the craft toapproach asymp- .ftotiallyxgandthereafter maintain the Ydesired Hight path. .Aspointed out above, in the'systemfof myabove mentioned patents, craft rate of turn .signal is obtained from aavertical gy'roscope whichmeasures craftbank angle. This bank angle signalwhile being generally propor- .,ztonal i'to; craft vrrate :of turn is valid only if the turn is Yproperly Ycoordinated 'and' the airspeed is maintained fsuhstant-ially constant. Furthermore, the Avertical gyro l Vquired for m'easuring'craftbank angle -is' not onlyrelalyfeip'ensive" but is V'also rather large and bulky as Y withhfor'example, a rate gyro. Therefore, y* lccp'naiice with anh'object Iof the present invention e Yvertical"gyrois eliminatedandV a headingratesignal i isugeneratedjby means., of va'mrate -'oftu'rnjgyro, which sures craft h'eadinglrate directlya'nd thereforepproof'thepresent invention this latter Ysignal is derived by radio-denedbeam 'andV thereafter to cause'the craftltio defining means isernployed. Under Vsuch conditions'ac.-

tl ,h owz rnuch correction in attitude should take s j'videdlfor switching out the radionavigation receiver 'Il 5. 'tior`1ed`V Patents 27,613,350 or 2,613,352 and comprises,

lingmto beV preselectedva further signal generator such'fillard ILS localizer signals or VOR signals or the like,` 5

provides a direct' current signal proportionalv to'y thek horizontal displacement or lateral displacement of the'CY craft from a radio-defined flight path. ThisY signal'i modulated and amplified in a suitable modulator ampli#` er llfthe output fof which is fed to a conventionafi signall imiting device 1.2 and also to a lagf'compensate radio rate signal generating device 13. .The= limiter 12? is provided to prevent the displacement signali from obtaining complete control of the Isystem when the" di placement of the craft from the radio-defined patl-'i sj very large and the limit imposed thereby serves to deter?" mine the intercept angle that the` craftv makes with tlv.V radio denedfpath as described in detail in the secondof my. above mentioned patents. Thelag compensate rdi rate :signal generating device 13 provides an outputv proportional to the rateo-f approach on the craft Vtoward theradio beam and is preferably ofthe type illustrated intheabove'noted application Serial No. 336,668.V The' outpt'f tHe-latter deviceis combined with fthe output of limiter 12 and limited 'as b'ya conventional-.limiter 1i the' output of which is fed to a summin'gfamplier =15. yIn amplilier f 15 'the 'signal vrepresenting the sum'fofV-thre displacement and displacement rate of the. crafewitb vrespectftothebearnlis algebraically combined with@ s'ignalrepres'entative of the"'acceleration ofthe craft'witli respect to the beam. In accordanceY withthe teachings ta device which is directly responsive to and provides measure'of the actual rate of change of craft heading lrather/than bya deviceV which senses roll angle as .inniy above-mentioned patents. This'rate of change of heatv ing of 'the craft is proportional tothe acceleration of the Acraft withrespect to the beam. Thus, the output of amplifier 15 is proportional to the sum of beamirate, ldisplacement, and acceleration-and is applied through-a 'suitable'demodulator 16 to a conventional D. C. meter `movement 17 which 'in turn positions the' verticalrneedle 18zor'pointer; of ani-indicating device 19 to-indicate any departure of the cra-ft fromv thel desired asymptoticv apfproachpath. The' meter 19. is preferably of the type described 'in `rny above Vmentioned patents. Thus, `by controlling theicraft to keep 'the algebraic sum ofthe foregoing terms equal tozero, the pilot willl'cause )craft to `ilyan asymptotic approach path,` toward the ybe:maintained on'the beam. n Y *K .A AAs in the above noted patents, vthe apparatusV illustrated vin Eig. l `maygbfeusel forcross countryV dead reckoning .,nayigation of the aircraft wherein no radio grond track vcurate headings may be maintainedV and new heading, ,may be asymptotically approached and thereafter m i" tained `by*` simply maintainingA the indicator 18 'at la' deliected position. For this'purpose a switch 20 is` 'prio-r.

Ajalndlthev radio, rate unit'13` and 'in its place insertingl'fg: n .heading signal derived fromany conventional heading si'g'nal source r12,1.. Preferablyv thisI heading signal source is of the`foi'm described in either `of 'my abovern "generally,"a"gyromagnetic compass 50 having a suitable4 `,sig^n'alfgeneratorsuch asa synchro 51 which suppliesg ff', Arial"epr'esentative1of, the heading'of the Aaircraftf'relm "tiveito'theirnagnetic north." For enabling adesiredy the. heading of the craft from the heading on which it 'is .ying, the pilot selects the desired new heading on heading selector 53 and a turn command signal representative xof the error or difference between the existing heading 'and the desired heading is generated in the stator of :synchro 52 and when the actual heading of the craft, as vmeasured by gyro 50 and synchro 51, corresponds to that lselected, the outputof `synchro 52 is reduced to zero.

In the illustrated embodiment of the present invention a nrate gyroscope is employed as a device for measuring the rate of change of heading of the craft although it will be understood that other forms of rate measuring devices may be used. Itis desirable in regards to aircraft performance tto render the gyro quite responsive to craft heading rates. However, under such conditions of low restraint and low damping the gyro is extremely sensitive to high frequency noise i. e. to random perturbations in craft heading rates produced by wind gusts and the like. These high frequency perturbations or noise may be removed if the signal output of the gyro is passed through a filtering device such as a low pass filter. Unfortunately, however by passing the output signal of the gyro through such a low` pass filter the time lag produced by the filtering action thereof impairs the usefulness of the signal. However, in accordance with the teachings of the above noted application the lag of the filter may be compensated for by deriving a signal having a leading phase characteristic with respect to the noisy primary signal. In the apparatus of the present invention this source of leading signals may be obtained by inclining the precession axis of the rate gyro in the plane including the craft longitudinal and vertical axes, thus rendering the gyro sensitiveboth to craft rate of change heading and craft rate of change of roll angle, the latter signal being proportional to the accelera- '.tion of craft heading. This added signal component is .therefore filtered through the same filter as the heading .rate signal and is used to compensate for the lag in the lter.

Accordingly, I have replaced the vertical gyro of my above noted patents with a rate gyro indicated generally .at 25 which comprises a rotor 26 mounted in a gimbal 27 for spinning about a normally athwartship axis y. The gimbal 27 in turn is pivotally supported in the craft for precession about an axis p inclined at an angle in the vertical plane defined by longitudinal craft axis x and the vertical axis z. Restraining springs 28 acting between gimbal 27 and the aircraft frame 29 `are selected such that the gyro has a relatively high sensitivity or response to craft rates of turn and rates of roll. Mounted on the precession axis p of rate gyro 25 is a suitable pick off device or signal generator 30 which may be the selsyn type which supplies an output signal proportional to the magnitude and direction of the precession of gimbal ring 27 in response to" craft rates of turn and rates of roll. Suitable damping of the gyro may be provided by means not shown ,having a relatively light damping characteristic i. e. an :amount sufficient to render the gyro insensitive to very high frequency rates such as those produced by craft ".vibration.

Thus, with gyro 25 mounted in the aircraft in the position illustrated in Fig. l, it is responsive both to the rate ,of change of heading of the craft and also to the rate of ;roll of the aircraft, the rate of roll responsiveness being Ldependent upon the angle of inclination 0 of the precession :axis p. Also, the signal generator 30 provides a signal having components proportional to both of these craft zrates. The output of signal generator 30 appearing at lead 31 is applied, after demodulation by suitable demodulator 31', to the input of low pass `filter 32. Low pass iiilter 32 is preferably of the RC type the values of R and @C being selected such as to have a time constant that will pass only the desired low frequency components of the composite rate of turn signal and will suppress or attenuate the undesired high frequency components thereof.

The output of low pass filter 32 is modulated as by suitable modulator 32', and applied to the inputs of A. C.

mixing amplifier 15 through lead 33. The input signal t'd amplifier 15 proportional to beam acce1eration,`` in the case of a beam approach, or to heading rate, in the case of a dead reckoning flight, is a smoothed version of a generally noisy rate of turn signal having the required phase relation With respect to the heading signal from the source 21 and 180 phase relation with respect to the signal derived from a localizer receiver 10.

It will be understood that the term phase as used in the present discussion is used in a sense that in a servo system where a signal is provided tov represent the rate of change of the displacement of an object from a desired condition, such a rate signal is said to`have a leading phase, system-Wise, relative to the phase of the signal representing the displacement itself. Likewise, a-signal representing the rate of change of the rate .i signal, i. e. an acceleration signal is said to have a phase, system-wise, which is displaced from the signal representing the displacement signal. Hence, where a system is designed to use a displacementsignal and its time derivatives, such as rate and acceleration, these derivative signals are useful as such only when they have the proper leading phase relation with respect to the displacement Signal. If the derivative signals have any other than a desired phase relation their usefulness decreases.

From inspection of Fig. 2` it will be seen that for compensation of the lag produced by filter 32 `the proper amount of heading acceleration or yroll rate signal must be used; specifically, the amount of roll rate Signal inlthe composite signal from pick olf 30 is determined bythe phase lag produced by the filter 32. In other Words, the. angle 0 that the precession axis p makes with the fore and aft axis r of the aircraft is related to or is a function of the lag 0' produced by the filter 32.

Since the relation between roll rate and heading acceleration depends upo-n air speed, compensation would occur only at one air speed for any particular angle `of inclination 0 of the gyro precession axis.

In Fig. l the gyro 25 is shown as being inclined at a fixed angle. Under this condition the amount of lag compensation signal available at extreme low and high air speeds for many representative aircrafts would vary between .65 and 1.35 respectively of that required for exact compensation. It has been found that the operation of the craft under these conditions is completely satisfactory. Where exact compensation is desired the angle of tilt of the gyro 2S may be continuously varied in accordance with the air speed of the craft. Such a modification is shown in Fig. 3 whereinthe gimbal 27 is pivotally supported in a frame 35, the frame in turn being pivotally mounted in the craft for rotation in about the craft athwartship or y axis. The frame 35 may be continuously positioned as a function of craft air speed by means of a conventional follow-up servo loop coupled between the gimbal 35 and a means responsive to craft air speed such as a conventional air speed sensor 36. Since the heading rate sensitivity should be increased with increasing air speed the angle 0 should likewise be increased with increasing air speeds so that more roll rate or heading acceleration signal will be generated in pick off 30 with. increasing craft speeds.

In some applications it may be desirable to compensate the output of rate gyro 25 for errors in the rate of turn measure which are produced by banking of the craft since gyro 25 is unstabilized and measures rate of yaW rather than space rate of turn. Forthis purpose a vertical accelerometer 55 which provides an output in accordance with accelerations of said aircraft along the crafts vertical axis may be employed, the output of which is used to energize the signal pick-off 30 in the manner disclosed in U. S. Patent No. 2,602,239 to Wrigley, which patent is assigned to the same assignee as the present invention.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing frbrnthe scope phereof itis intended that all .matter contained in the -a-bove description or Yshown in the accompanying-'drawings shall be interpreted as illustrative and not'linia limiting sense.

vWhatis claimed is: '-1. A navigation system for aircraft comprising means for providing aiirstv signal the'representative of the difference betvveen an existing craft heading and a selected heading, means Afor providing a second signal proportional to thefriate of change of craft heading, said second signal providing means comprising means lfor measuring the rate of'jhange of r4heading and the rate of roll of said craft andfor vsupplying signal components proportional thereto, said :signal components-including both the high .and low Ys'frequencies'ofboth of said craft rates, means responsive f tsaidsigxialcomponents for providing an output signal proportional only to the low frequency components thereofsaidf output .signal constituting said second signa ,andfrnearis'vrespons'ive to said first and second signals forproviding a craft control signal varying in accordance with lthe algebraic sum thereof.

"2. .Ina craft Ynavigation system, the combination com- 'prisingarate of -turn responsive device mounted in said aircraft-insuch-a position that it is responsive both to the rate of .change of craft heading and the rate of change of 3cr-'aft roll angle, signal generating means Vcoupled there- `i'vith iforfp'roviding a signal having components proportional lto=saidrate of heading and rate of roll of said craft, saidS-rateof turn'lresponsiv'e means and said signal generating Arneansibeing sensitive to both the desired low frequen'ey components and the undesired high frequency ompoiientsoffsaid rates of craft movement, `a low-pass lter'i-'lconnected to -said signal generating means and responsive to the signal produced thereby for passing said frequency components and for attenuating said high frequencylcomponents of said signal, the mounting position of said device being related to the time constant of said filter. 4

3; Apparatus -for providing a Vmeasure of the rate of changeof heading of an aircraft comprising a rate of turn gyroscope mounted in said aircraft with its axis of prelcessionfinclined in a plane including the crafts longitudial 'aridvertical axes vwhereby said .gyro is responsive both 'to the heading rateand the roll rate of the'craft, the restra` A.gforce'onf-said gyro 'being such thatit Will have arapidL response ,to said rates of craft movements, a signal generator means mounted on said precession axis for-providing a 4signal having components proportional vtrfrbotll[heading rate and roll rate, said components inriinglJotljr, theV high kand y10W frequencies of said rates f craft movements, and@ low-.pass filter connected to regnal produced by said signal generator for providingan .output 'signal :including only ,thelowirequency signal generating means.Y

components thereof, .the .angle of .inclination 4of Ps'aid ,gyro Y being proportional tofthe time constant Jof saidlter -whereby said output signal provides ameasure'` of t thefrate 'of change Aoffheading of said crafthavingahigh response and at the -sametiinc arhigh degree ofstability. it. Apparatus as .setfort'h in claim 3 wherein said rate of turngyrosc'ope is niountedin said aircraft'wsothatfits angle ofiinclination 'is l 'adjust'apble,'and means 'responsive p 'tothe air ,speed of ysaid aircraft for adjusting said .angle of inclination* 'Y -5'.`Apparatus assetfforth-in claim 3 furtherincluding j means responsive to accelerations of said aircraft along l the craft-s vertical axis, and means responsive lto Ysaid last) inentio'ned means' for ,inodifyirg'the signal outpuoffsd 6." 'navigatioii systemtfor aircraftgcompi'isingmeans 1 for supplying'a signal proportional to the displacement of f the 4craft from a -radio-beamdefne'd course, means' for providing ya signal proportional tto the" rate of approaim of said craft-toward said course, means for providing'a signal porportional to the acceleration of'said craft toward said Y,coiirse y,inchi'ding' means responsive to `the ra'tef v, lianes ,of Craft-heading and Vfor,`t 1f i11ing` asignaln portional thereto, .said last-mentioned rneans'ibeing` fr 'spons' e Aboth to the desired low Vfrequency componen ,of heading rate and the undesired high frequency"do".x y ponentsthereof and ,said signalproportional' thereto 1 including said Vfrequency components, filter means con.` nected ltorreceive rsaidsignal .and having a .constantV f lsuch thatsaidpdesired'low frequencycomponents vvill'b;V A .passedand saidlund'esired highfrequency componentsfivill Vbe qattenuated, said heading rate responsive means being ,Y mounted in aiposition in )said aircraft, such that it will also respondtolrates of craft roll, the mounting position of saidrategresponsive means being related to thetirne constant of said filter, and ineans responsiveto said 'dis placement, rate, and acceleration vsignals for supplying .anoutpusignal in accordance with the algebraic sinn thereof.A 7. A navigation system asset forth in .claim wheren saidi'neanslresponsive to the .'rate of change of craft headirolrnprises a rate of .turn gyroscope having its precesl sionfaxis positionedeinsaidcraft .in an inciined orientatilvlnvqinY the planedeined by `thevcraft foreand-aft,and vertical l`axes', the angle ofinclination thereof being ,pi-oportionalto the time constant of `said lter. 

